The land is becoming more and more expensive these days and building regulations usually don’t allow going beyond ground plus one story so, people have no option other than to build a basement in order to increase the covered area. This is not all; a basement also provides a firm footing to the house in unstable soil found in areas like reclaimed land near the seashore or land filled wet lands So, basement infect has a dual purpose.
But building a basement in a region where the underground water-table is hardly a few feet below ground is like going against the nature. As they say in English; "when you go against nature, the nature kicks back" so, basement construction should be taken seriously or else the damage is sometimes beyond repair. I would also like to clear one more thing here that; in Karachi, the underground water-table has no relation with distance from sea as most people think. As a matter of fact, water-table in populated areas is caused by leaking sewerage lines and that is the reason why old areas of Defense Society have a higher underground water-table, as compared to areas under- development just few hundred meters away from sea.
Constructing a basement requires extreme precautions otherwise, a leakage is almost inevitable. The water pressure on basement bottom and walls is so immense that even a small flaw in concreting may cause a breach. We know it from the Archimedes Principle that; the water-pressure upon the exterior of basement container (raft and walls), is equal to the weight of water displaced. Just imagine the volume of basement and the weight of water that has been displaced by it and you can have some idea, of the pressure. This in fact is the same pressure that keeps a ship loaded up to the throat with cargo, to float on the surface.
Similarly most people don’t have any idea, as to what is most important precaution while constructing a basement. A lay-man thinks that an expensive and imported Waterproofing Chemical will ensure a leak proof basement, but that is hardly true. In the following, I will try to list the precautions in accordance with priority level.
De-watering is the key to waterproofing as if the concreting is done over a dry platform; there are very few chances of exposure to water and thus any leakage. The concrete requires that; it gets dried for once after the pouring and then any water will only help in curing process. But if for some reason; water keeps touching the concrete, it does not allow the concrete to dry for that one time and thus; remains unset. Such points in the raft or retaining walls are very difficult to waterproof, even with the most advanced waterproofing chemicals. Therefore; extreme precautions should be taken in this regard. In my career as a Waterproofing Consultant, the worst basements I saw, were the ones where this specific precaution was not taken.
Usually in a de-watering arrangement; pvc pipes with netting wrapped around the lower ends are put in the soil vertically and are connected to a motorized suction system, but this system is very vaguely installed and can fail due to loosening of a joint. It is always better to have back-up plans like making a canal around the basement at least 2 Ft. deep to ensure that water does not rise up to the raft. Furthermore it is advisable to make two wells (4 Ft. deep and 4 Ft. wide) diagonally opposite in the basement pit. This will not only ensure a visible underground water-table, but also may allow to remove water manually, just in case the suction motors fail for some reason during the pouring process. The canal made around basement is connected to these wells.
I recommend all three procedures as more precautions ensure more protection. Always remember that in case of faulty concreting; once the water enters concrete and touches the steel net, a slow cancer like process starts in which the water travels in the rust peel produced over the surface of steel. In long run this may cause damage to the integrity of the structure due to decomposition of steel. Prevention is better than cure especially if the cure is only symptomatic relief. That is; even if the seepage is stopped from inside the basement by application of Crystalline Waterproofing Material, this damage continues to grow and the the waterproofing may only provide dry surface inside the basement but not fix the problem completely.
As a matter of fact, the concrete itself is Waterproof and if the concreting is done properly by the book, it cab be waterproof without the use of any waterproofing products. Having said that, in no way I am suggesting that water proofing chemicals should not be used as they are always helpful to fix any flaw left during the entire process or any cracks produced after a seismic activity.
Mix design is very important when it comes to concreting. A proper mix design is one which helps achieve maximum compact concrete. A simple way to check a mix design is to mix all the components in given proportion and then take a sample in a glass to see if the mix design is compact. If you can see spaces than remember that no water proofing chemical can fix the concrete. Waterproofing chemical can only fix capillaries under (01 mm) in size and any crack or void more than this value cannot be fixed by any chemical.
In large projects; graded crush is used to ensure that voids may not be created in the concrete. In small projects like residential houses; graded crush is not used normally. What can be done is that in the first pouring which should be Monolithic, up to 3/4” crush can be used. But in the next pouring to complete the walls up to 9-10 feet, the crush size should be reduced to 1/2”. Large size of crush causes voids.
The sand should be of good quality and free from dust as well as large stones. The best way is to get the sand filtered by a 1”-(1.5)” net to remove dust and large stones. Large stones will not pass through the net and dust will be blown away by the wind during the process. The sand must be grainy such that it has grains of different sizes. Always ask an expert to buy the sand as he will check the truck before purchase.
What is important here is also to maintain the right water content so that the concrete is neither too much thick nor too much thin that water gets separated. The concrete should be such that it may not have loose water as this loose water has all the cement juice and water proofing chemicals. Proper consistency ensures equally spread cement content in the concrete. The term used by contractors is that it should be like “Rabri”, a popular sweet dish in Indo-Pakistan made from cream and sugar which is exactly of same consistency as is required in the concrete during pouring. Since the people responsible for concreting are sub-contractors of the contractor for the day's work so, they have no interest in the strength of the concreting. They are answerable to no one and there prime interest is to finish the job as early as possible and run home. The excess water makes the job easy and complete in less time.
The ratio between crush, sand and cement also plays a vital part as more crush may cause voids. In case of ready mix concrete, the crush is more than sand but in manual concreting more sand and less crush is advisable.
Where to have a structural Joint
In the past; the raft/floor was made in one attempt and in next; the walls were concreted. But with the passage of time it was realized that in such procedure, the joint of walls with the raft usually leaks. More over pouring of concrete in walls up to 9 feet in one go causes honey combing at the lower section of walls. Therefore Monolithic pouring was introduced in which walls up to 3 or 4 feet are poured along with the raft. Then a rubber like strip called Water-Stopper is placed in the center of the approximately 9" wide wall all around the basement. Then the rest of the wall is poured.
Water Stopper is a (2cm x 2cm) strip which is placed in the middle of structural joint. It expands with water exposure caused by freshly poured concrete. The expansion of this strip is irreversible. It develops a gasket between two different pourings and seals the joint. This is a very useful Water Proofing component and in no case should be avoided. Always remember that the most vulnerable points of leakage in a basement is usually the structural joint.
Multiple points of pouring while concreting
Again to make his job easy the sub contractor responsible for pouring tries to have minimum pouring points and by making concrete running, he will try to pour all the concrete from minimum points.
This would save him from using time and labor in changing the points again and again. What happens in this scenario is that the crush stays but the liquid slurry spreads and thus making the mix design go out of proportion. As a result honey combing is produced in the concrete as points of pouring are left with higher concentration of crush.
Non Stop Pouring
In hot regions, if the pouring is stopped for even half an hour, the concrete gets dry and a diagonal joint between previously poured concrete and freshly poured concrete is produced. Such diagonal joints on walls are often the cause of leakage. The picture above also shows one such point. Usually such joints are either produced if the concreting is not done in a planned manner and one point is left for too long or the lunch break becomes too long.
This is also one of the most important factors to achieve a concrete with greater compaction and strength. Vibrator helps remove voids caused by air pockets from the concrete. Usually the contractor will appoint a separate person for manual “rodding”; a procedure in which pvc pipe is used to make sure that concrete is properly filled the far corners of shuttering leaving behind no air-pockets but this procedure is not much effective. A Vibrator should be used by an expert person as over use for few seconds may cause swelling of wooden shuttering even opening of shuttering. The Vibrator should be used keeping the shuttering contractor along so that he may always tell when to stop. A Vibrator is a 2 feet long iron rod fixed at the end of a rubber pipe which is run by a petrol engine to create vibration in the freshly poured concrete. The use of Vibrator helps achieve protection against leakage by having compacted concrete. Compact concrete also adds to the PSI of the concrete means more stronger structure.
Plugging of holes in wooden shuttering
If the holes in wooden shuttering are not plugged which are usually plugged by paper (cement bag pieces), it would allow the water of concrete to spill out. This water is like the juice of concrete and must remain embedded in the concrete. Thus concreting of basement is a team effort in which all players have to perform their duty in order to have a leak proof concrete.
Use of steel spacers to prevent collapse of shuttering
Spacers are placed to make sure that the wall thickness is not reduced at any point. Always use steel spacers instead of wooden spacers as wood decomposes due to bio-degradation and creates holes in the concrete in long run.
Best way to Concrete
"The best way to concrete is off course to use steel shuttering and ready-mix concrete but since it is expensive and most people don’t understand its benefits, unfortunately manual concreting with wooden shuttering is still the way to concrete in Pakistan. If ready mix is used, than much of this discussion is not even required".
Which Water Proofing System?
This is a whole topic in itself and even a controversial one. Some Architects are still using the old Poly-bitumen Membrane System to fully enclose the basement container. I strongly recommend Built-in or what is called the Integral Waterproofing System (Crystalline) as it is more foolproof as less human error is involved in it.
I explain in detail now as to why; in case of 4 mm Poly-bitumen membrane, what they do is they melt join 4 ft. wide sheets with the help of LPG torch. Just imagine that while melt joining all the joints; what may be the chances that a small hole or many are left. Infect; it is hundreds of feet of joint that has to be sealed with a torch operated by a non qualified labor. What happens is that holes is always left. Once water enters from this hole, its only a matter of time before it surrounds the entire cavity around basement. This may take one or two years. The cavity I am talking about here is the cavity between the membrane and the concrete all around the basement exterior. Keep in mind that this water causes same amount of pressure as it would have caused in the absence of membrane. Thus the whole jacketing procedure becomes useless in one or two years and if the water does not come inside the basement, the credit may go to good concreting instead of membrane jacketing.
On the contrary the built-in waterproofing is a powder which is mixed in the concrete at the time of pouring. It is Crystalline Water Proofing Material and forms tiny crystals inside the concrete to seal all the capillaries under 1 mm in width. The crystal it produces are breathable and allows the curing process to complete as usual. This chemical is like a magic as it stays dormant inside the concrete unless a crack is produced by either settlement or a tremor. As soon as water enters from a crack; the chemical reacts and forms crystals. Infect this chemical consumes that water and turns into water of crystallization like sugar crystal but the difference is the crystal it produces are irreversible crystals unlike sugar which can easily dissolve back into water.
After concreting; Precautions
In hot regions like Karachi, I suggest that a thick jute cloth should be used to cover the concrete as soon as the shuttering is removed. This cloth should be kept wet to ensure a proper curing process of the concrete. Especially in hot months of April, May, June and September as in these months the water evaporates immediately. Sprinkling of water to keep the concrete wet all the time for at least ten days helps in proper curing of concrete. Concrete attains its full strength in 28 days and proper watering during first ten days is the key to achieve desired strength.
Cutting of Steel Strips used to bind the shuttering
The steel strips used to bind the shuttering should be cut up to at least half an inch deep into concrete after two weeks of concreting and then plugged with mortar (mixed with some Crystalline Waterproofing Material and SBR). These steel strips rust away in long run and creates a channel for water to come inside. Infect this is why steel shuttering is so useful as it does not require steel strips to bind it. The same process should also be done at exterior of basement.
Plaster or Membrane Coating on Exterior of Basement
I always recommend membrane coating on the exterior of basement instead of plaster. Again there is a logic. Plaster is not waterproof and even if we apply membrane coating over plaster, there may be some point left for plaster to absorb water. Once plaster absorbs water, it spreads to the entire plastering around the basement. In case of membrane coating, even if some point is left while coating, water may only touch the concrete in that part and not affect the entire exterior of basement container. The following article explains the above.
A coat of Crystalline Waterproofing Compound from Negative Side
It is recommended that after the structure has been completed till the top and settlement if any has taken place, a coat of Crystalline Waterproofing Compound should be applied. It is applied from inside (Negative side) with nylon carpet brush after making a slurry with water. This will further protect against any water infiltration in the basement. This crystalline waterproofing compound is infect one of the most wonderful inventions in building industry. Remember that this process should be done before even plastering the roof on bare concrete as cement remains from roof plastering left on walls and floor, may affect the absorption of this chemical.
Plastering in the End
The basement should be plastered in the end to make sure that there is no leakage arising due to settlement of the structure. Usually the plaster activity is started from top moving downwards.
ajit on August 31, 2019:
useful, and very lucidly explained, students of engineering, architecture and building science would benefit a great deal
WAhmad on August 07, 2016:
You didn't mention cost/sq ft if building basement as per this article.
Any ideas on cost at least just material cost would help.
Tufail Shah on October 09, 2015:
Dear Arshad Bhi, i constructed my basement. it is half way through. i read your precious article and now i need to discuss certain point with you.
can i get your id or phone number.
will really appreciate.
Muhammad Ajmal on May 17, 2015:
excellent, very useful for home building
Wasif on January 17, 2014:
Brilliant article .. Thanks a lot
Bill on December 05, 2013:
Basement waterproofing is really important. I hope that everything worked out for them. http://www.safeguardwaterproofing.com